A. Field
The present invention concerns a device for adjusting the operating pressure of an oil-injected compressor installation.
B. Related Art
From EP 0.942.173 in the name of the same applicant is already known a device for adjusting the operating pressure of an oil-injected compressor installation which is provided with a compressor element that is driven by a motor with an adjustable rotational speed, controlled by a control module, whereby said compressor element is provided with an air inlet and with a compressed air outlet onto which is connected an oil separator with a compressed air pipe for supplying compressed gas, whereby the device is provided with a controlled inlet valve which is connected to the above-mentioned air inlet and a blow-off mechanism with a blow-off pipe connecting the oil separator to the inlet valve and which can be closed off by means of a blow-off valve.
In such a known device, the inlet valve of the compressor element is pneumatically controlled.
A disadvantage of such a pneumatic control system is that there is a continuous loss of compressed air, which is necessary for the good operation of such a control system.
Another disadvantage of such known pneumatic control systems is that the operating pressure of the compressor installation is always higher when it is unloaded than when it is loaded, as a result of which the operating pressure requires more power from the engine when the compressor installation is unloaded.
Another disadvantage of the known pneumatic control systems is that the regulating pressure pipes and air chambers create large time constants, such that in case of sudden fluctuations in the outlet flow of the compressor installation, there will be “overshoots” or “undershoots” in the operating pressure, whereby this operating pressure will suddenly represent a very high or very low value respectively.
A disadvantage connected thereto is that when the dimensions of the regulating pressure pipes are altered, for example due to a replacement or a repair, the above-mentioned time constants will assume a different value, which is disadvantageous to the stability of the adjustment.
An additional disadvantage of the known devices is that condensate may be formed in the regulating pressure pipes of the pneumatic control system which is discharged via air holes while the installation is operational, but which, after the compressor installation has been turned off, remains in the pipes and may accumulate there.
Also, in case of temperatures below zero, the regulating pressure pipes may freeze up and thus prevent the good working order of the pneumatic control system.
Another additional disadvantage is that with the known devices, the required operating pressure is set manually by screwing down a pneumatic regulating valve. Moreover, it can only be set when the compressor installation is operational.
Another disadvantage of the known devices is that the inlet valve usually has the shape of a piston valve which is disadvantageous in that its design causes large inlet losses.